Various types of PM sensors (particulate matter detection sensors) for detecting the amount of PM exhausted from an engine or the like have been proposed. For example, a PM sensor disclosed in JP 59-196453A (corresponding to U.S. Pat. No. 4,656,832) includes a pair of opposed electrodes on an insulating substrate. The accumulation of PM changes a resistance between the pair of the electrodes. By using this property, the PM sensor is configured to detect the amount of PM by measuring the resistance between the electrodes. In this case, a signal output circuit connected to a sensor element forms a voltage-dividing circuit configured by a resistance between the pair of opposed electrodes and a predetermined shunt resistance. The signal output circuit is configured to output a voltage at an intermediate point of the voltage-dividing circuit as a sensor detected signal.
A sensor detection value obtained from the PM sensor often deviates from a normal value due to the deterioration of the PM sensor with time, or the failure or the like of the signal output circuit connected to the PM sensor, thereby disadvantageously resulting in wrong detection of the amount of accumulated PM. In such a case, the wrong detection of the amount of accumulated PM adversely affects various types of control processes to be performed using the result of the detection.